A method for recycling and activating waste graphite and its application

Abstract

The present invention discloses a method for recycling and activating waste graphite, comprising the following steps: 1. Conventional recycling of graphite; 2. Pyrolytic removal of conductive carbon and binder; 3. Two-step activation method for recycled graphite; 4. Post-treatment of the calcined reaction product, whereby hierarchical graphite nanosheets can be obtained. The hierarchical graphite nanosheets are used as a sulfur carrier for the positive electrode material of a lithium-sulfur battery, with an initial capacity of 1410 mAh·g-1 at a current density of 0.1C; after 100 charge-discharge cycles, the remaining capacity is 841 mAh·g-1, and the capacity retention rate after 100 charge-discharge cycles is 59.6%. The present invention has the characteristics of low raw material cost, simple process equipment, and simple waste liquid treatment; it realizes the adjustment and reconstruction of the microstructure of graphite - etching from a blocky structure into a hierarchical nanosheet structure, thereby improving the performance.

Description

Technical Field

[0001] The present invention relates to the field of waste lithium-ion battery recycling, and particularly to a method for recycling and activating waste graphite and its application. Background Art

[0002] The lithium-sulfur battery has an ultra-high theoretical capacity of 1675 mAh·g -1 ; based on an average discharge voltage of 2.15 V, the theoretical energy density can reach 2600 Wh·kg -1 , which is much higher than the theoretical energy density of traditional lithium-ion batteries. The rapidly growing application of lithium batteries has directly led to the problem of waste lithium-ion battery recycling. Since the positive electrode is the most expensive part of the lithium battery, the current waste lithium-ion battery recycling mainly focuses on the positive electrode, and there is still a lack of relevant research on the recycling method of the graphite negative electrode. Currently, the conventional treatment method for the waste lithium battery negative electrode is to incinerate the graphite together with the cathode during pyrometallurgy. This method directly leads to the generation of a large amount of greenhouse gas carbon dioxide. To recycle high-purity graphite negative electrode materials, the currently common methods include direct physical recycling, hydrometallurgical recycling, extraction, electrochemistry, etc.

[0003] In the currently conventional organic solvent method for graphite negative electrode recycling, such as in the existing literature 1 (Yang X, Li R, Yang J, et al. A novel route to constructing high-efficiency lithium sulfur batteries with spent graphite as the sulfur host[J]. Carbon, 2022, 199: 215 - 223.), after the graphite is first peeled off from the negative electrode copper foil, the binder polyvinylidene fluoride is removed using an organic solvent, and thus the recycling of the graphite negative electrode can be achieved. The problems existing in this technical solution are as follows: the organic solvent is N-methylpyrrolidone, which itself is toxic and the raw material cost is expensive, directly resulting in an increase in the cost of the entire technical solution; moreover, there is also the problem of waste liquid treatment. In addition, the recycled graphite has a block structure. When it is reused as the positive electrode of the lithium-sulfur battery, most of the sulfur can only be wrapped on the surface of the graphite in the form of a single substance, resulting in a significant reduction in the sulfur utilization rate and cycling performance.

[0004] In addition to the organic solvent method, there is also a method that combines sulfuric acid curing leaching and high-temperature calcination to achieve graphite recovery. For example, in the existing literature 2 (Gao Y, Wang C, Zhang J, et al. Graphite recycling from the spent lithium-ion batteries by sulfuric acid curing–leaching combined with high-temperature calcination[J]. ACS Sustainable Chemistry & Engineering, 2020, 8(25):9447-9455), the purity of the recycled graphite achieved by the method of combining sulfuric acid curing leaching and high-temperature calcination can reach 99.6%. However, there are two problems with this technical solution:

[0005] 1. During the charge and discharge reaction process of the lithium-ion battery, lithium ions in the electrolyte repeatedly intercalate and deintercalate in the graphite lamellae, resulting in the structural collapse of graphite particles. Therefore, the recycled graphite obtained by this technical solution needs to be continuously calcined at a high temperature of 1500 °C to restore the graphite lattice, so as to achieve the purpose of restoring the graphite structure. This process directly leads to the problem of high energy consumption, resulting in the cost of recycled graphite being higher than directly purchasing commercial graphite, and unable to meet the requirements of commercial applications;

[0006] 2. The recycled graphite obtained by this technical solution still presents a block structure and is suitable for use in lithium-ion batteries. However, when the recycled graphite is used as the cathode of a lithium-sulfur battery, there are the same technical problems as those in the above-mentioned existing literature 1.

[0007] In short, this technical solution not only fails to meet the requirements of commercial applications in terms of cost, but also is not suitable for lithium-sulfur batteries in terms of material structure.

[0008] In addition, there is also a one-step high-temperature calcination method. For example, in the existing literature 3 (Yang J L, Zhao X X, Li W H, et al. Advanced cathode for dual-ion batteries: Waste-to-wealth reuse of spent graphite from lithium-ion batteries[J]. eScience, 2022, 2(1):95-101.), the binder is removed by high-temperature calcination of waste graphite. The problem with this technical solution is that since the binder is a carbon-containing organic substance, it will carbonize to form a large number of amorphous carbon spheres during high-temperature calcination and coat the surface of the graphite, resulting in the blockage of the pores between the graphite lamellae, thereby reducing the insertion of sulfur elements when used as the cathode of a lithium-sulfur battery and reducing the performance of the lithium-sulfur battery.

[0009] Therefore, according to the inventors' research, the technical solution for graphite recovery should be to use pyrolysis to remove the binder, followed by activation treatment - which can not only remove the amorphous carbon balls left on the surface after pyrolysis, but also etch the graphite to make it present a layered nanosheet structure. Ultimately, when it is applied to lithium-sulfur battery positive electrode materials, it can increase the specific surface area of ​​the material, thereby improving the utilization rate of sulfur and the electrochemical activity. Summary of the invention

[0010] The purpose of the present invention is to provide a waste graphite recovery-activation method and its application.

[0011] The activation method adopted by the present invention is to use potassium hydroxide as an activator, first perform solvent thermal activation, and then perform calcination activation. This can not only induce graphitization of amorphous carbon left after pyrolysis of the surface binder, but also etch the graphite block to make it present a layered nanosheet structure. The specific technical effects are reflected in the following two aspects:

[0012] 1. The raw material cost is low, the process equipment requirements are low, and the waste liquid generated during the preparation process only needs a simple neutralization reaction to be treated, that is, the waste liquid treatment is simple;

[0013] 2. Potassium hydroxide is used as an activator to adjust and reconstruct the microstructure of graphite - etching the block structure into a layered nanosheet structure, thereby improving performance.

[0014] In order to achieve the above object, the specific technical solution for realizing the object of the present invention is:

[0015] A method for recycling and activating waste graphite comprises the following steps:

[0016] Step 1, conventional recovery of graphite, placing the negative electrode sheet of the discarded lithium-ion battery in deionized water, performing ultrasonic treatment under certain conditions, then taking out the copper foil detached from the negative electrode sheet, filtering, washing and drying, and obtaining a graphite-conductive carbon-binder mixture containing graphite, conductive carbon and binder;

[0017] The ultrasonic treatment conditions in step 1 are as follows: the ultrasonic power is 700-1200W and the ultrasonic time is 1-5h;

[0018] Step 2, pyrolysis removal of the conductive carbon and the binder, using a pyrolysis method to remove the binder by pyrolysis. The specific pyrolysis method is to pyrolyze the graphite-conductive carbon-binder mixture obtained in step 1 under an argon atmosphere under certain conditions to obtain recycled graphite;

[0019] The conditions for pyrolysis in Step 2 are as follows: the heating rate is 5 - 10 °C / min, the pyrolysis temperature is 300 - 600 °C, and the pyrolysis time is 1 - 5 h;

[0020] Step 3, two-step activation method for recycling graphite;

[0021] Add 0.1 g of the recycled graphite obtained in Step 2 and 0.2 g of potassium hydroxide to 50 ml of deionized water to obtain a mixed reaction solution. Then, ultrasonicate the mixed reaction solution for 2 h and perform the first step of solvothermal activation. The specific method of the first step of solvothermal activation is to carry out a solvothermal reaction under certain conditions. After that, dry the solvothermal reaction product under certain conditions by blowing air to obtain a mixture;

[0022] The conditions for the first step of solvothermal activation in Step 3 are as follows: the reaction temperature is 120 - 180 °C, and the reaction time is 5 - 24 h; the conditions for blowing air drying are as follows: the drying temperature is 100 - 200 °C, and the drying time is 5 - 24 h;

[0023] After that, perform the second step of calcination activation on the mixture,

[0024] The specific method of the second step of calcination activation is to carry out calcination under certain conditions under an inert gas atmosphere;

[0025] The conditions for the second step of calcination activation in Step 3 are as follows: the heating rate is 5 - 10 °C / min, the calcination temperature is 500 - 900 °C, and the calcination time is 1 - 5 h;

[0026] Step 4, post-treatment of the calcined reaction product, namely the preparation of hierarchical graphite nanosheets,

[0027] The specific method of the post-treatment is to first perform ultrasonication and then stirring in a nitric acid solution; after the post-treatment is completed, perform centrifugation and drying to obtain hierarchical graphite nanosheets;

[0028] The conditions for the post-treatment in Step 4 are as follows: the concentration of the nitric acid solution is 8 - 12 mol / L, the ultrasonication time is 5 - 60 min, and the stirring time is 1 - 12 h.

[0029] The hierarchical graphite nanosheets obtained by a waste graphite recycling - activation method are used as a sulfur carrier for the cathode material of a lithium - sulfur battery, and the initial capacity at a current density of 0.1 C is 1410 mAh·g -1 ; after 100 charge - discharge cycles, the remaining capacity is 841 mAh·g -1 , and the capacity retention rate after 100 charge - discharge cycles is 59.6%.

[0030] Experimental tests were carried out on the characteristics and functions of the hierarchical graphite nanosheets obtained by the method of the present invention, and the results are as follows:

[0031] The SEM test results show that the graphite obtained after the recycling and activation of waste graphite is layered graphite nanosheets, presenting a nanosheet layer structure, and the layer gap between the nanosheets is 50 - 100 nm.

[0032] The cyclic voltammetry test results show that the sharp peaks indicate good reactivity of sulfur in the composite material, presenting fast reaction kinetic characteristics.

[0033] The cyclic charge-discharge test results show that the layered graphite nanosheets prepared by the two-step etching method can provide an initial discharge specific capacity of 1410 mAh·g at a current density of 0.1C. -1 After 100 charge-discharge cycles, 841 mAh·g remains. -1 The capacity retention rate is 59.6%.

[0034] Therefore, compared with the prior art, the present invention has the following advantages:

[0035] 1. The present invention first proposes to use the recycled graphite modified by potassium hydroxide as the cathode material of the lithium-sulfur battery.

[0036] 2. Using potassium hydroxide as the activator, the present invention etches the recycled graphite from the original massive structure into a layered nanosheet structure through a two-step activation method, greatly increasing the specific surface area of the material.

[0037] 3. Using the graphite from the negative electrode of waste lithium-ion batteries as the raw material, the prepared layered graphite nanosheets have good electrochemical performance, showing an initial discharge specific capacity of 1410 mAh·g when used in the lithium-sulfur battery, which is 728 mAh·g higher than that of the recycled graphite. -1 -1 .

[0038] Therefore, the present invention has broad application prospects in the development of cathode materials for lithium-sulfur batteries and the field of waste graphite recycling. Description of the Drawings

[0039] Figure 1 It is the scanning electron microscope image of the layered graphite nanosheets in Example 1 of the present invention;

[0040] Figure 2 It is the cyclic voltammogram of the first three cycles of the layered graphite nanosheets in Example 1 of the present invention;

[0041] Figure 3 It is the constant current charge-discharge cycle diagram of the layered graphite nanosheets in Example 1 of the present invention and the recycled graphite obtained in Step 1 of Example 1 at a current density of 0.1C;

[0042] Figure 4 It is the scanning electron microscope image of the recycled graphite in Example 1 of the present invention;

[0043] Figure 5 SEM image of the recycled graphite and potassium hydroxide calcined and activated in Comparative Example 1 of the present invention;

[0044] Figure 6 Constant current charge-discharge cycle diagram of the recycled graphite calcined and activated in Comparative Example 1 of the present invention at a current density of 0.1C;

[0045] Figure 7 SEM image of the recycled graphite and potassium hydroxide hydrothermally activated in Comparative Example 2 of the present invention;

[0046] Figure 8 Constant current charge-discharge cycle diagram of the recycled graphite hydrothermally activated in Comparative Example 2 of the present invention at a current density of 0.1C. Detailed implementation manners

[0047] The present invention further elaborates on the content of the present invention through examples in combination with the accompanying drawings of the specification, but it is not a limitation to the present invention.

[0048] Example 1

[0049] A method for recycling and activating waste graphite, comprising the following steps:

[0050] Step 1, conventional recycling of graphite. The negative electrode sheet of the waste lithium-ion battery is placed in deionized water and ultrasonically treated under the conditions of an ultrasonic power of 1000W and an ultrasonic time of 1h. After that, the copper foil peeled off from the negative electrode sheet is taken out and then filtered, washed and dried to obtain a graphite-conductive carbon-binder mixture containing graphite, conductive carbon and binder;

[0051] Step 2, pyrolytic removal of conductive carbon and binder. The binder is removed by pyrolysis using a pyrolysis method. The specific method of the pyrolysis method is that the graphite-conductive carbon-binder mixture obtained in Step 1 is pyrolyzed under an argon atmosphere at a heating rate of 5°C / min, a pyrolysis temperature of 500°C, and a pyrolysis time of 2h to obtain recycled graphite;

[0052] Step 3, two-step activation method for recycled graphite;

[0053] 0.1 g of the recycled graphite obtained in Step 2 and 0.2 g of potassium hydroxide are added to 50 ml of deionized water to obtain a mixed reaction solution. Then, the mixed reaction solution is ultrasonically treated under the condition of an ultrasonic time of 2h and then subjected to the first step of solvothermal activation,

[0054] The specific method of the first step of solvothermal activation is to carry out a solvothermal reaction under the conditions of a reaction temperature of 150°C and a reaction time of 12h. After that, the solvothermal reaction product is dried in a blast dryer at a drying temperature of 150°C and a drying time of 12h to obtain a mixture;

[0055] After that, the mixture is subjected to a second calcination activation.

[0056] The specific method of the second calcination activation is as follows: under the condition of an inert gas, calcination is carried out at a heating rate of 5 °C / min, a calcination temperature of 700 °C, and a calcination time of 2 h.

[0057] Step 4, post-treatment of the calcination reaction product, i.e., preparation of hierarchical graphite nanosheets.

[0058] The specific method of the post-treatment is as follows: in a nitric acid solution, ultrasonic treatment is first carried out, and then stirring is carried out. The concentration of the nitric acid solution is 10 mol / L, the ultrasonic treatment time is 30 min, and the stirring time is 2 h.

[0059] After the post-treatment, centrifugation and drying are carried out to obtain hierarchical graphite nanosheets.

[0060] In order to prove the microscopic morphology of the hierarchical graphite nanosheets, SEM tests are carried out; at the same time, in order to prove the influence of the activation in step 3 on the microscopic morphology, the recycled graphite obtained in step 2 is subjected to SEM tests as a reference.

[0061] The SEM test results of the recycled graphite are as Figure 4 shown. There are many impurities in the recycled graphite, resulting in the recycled graphite presenting a bulk structure. According to the common knowledge in the art, when directly using the recycled graphite obtained by pyrolysis as a sulfur carrier, it is difficult for sulfur to enter the interior of the graphite.

[0062] The SEM test results of the hierarchical graphite nanosheets are as Figure 2 shown. The hierarchical graphite nanosheets have a nanosheet layer structure, and the layer gap between the nanosheets is 50 - 100 nm.

[0063] By comparing the SEM test results of the hierarchical graphite nanosheets and the recycled graphite, it can be seen that the direct reason for the formation of this nanosheet layer structure is the two-step activation method of first solvothermal treatment and then calcination in step 3. According to the common knowledge in the art, the nanosheet layer structure of the hierarchical graphite nanosheets is beneficial to the loading of sulfur.

[0064] It should be particularly noted that through the inventor's research on the existing technical solutions in the art, it is found that there is no relevant technical report on the technical solution of using the two-step activation method in step 3 of the present invention to treat recycled graphite, converting the recycled graphite from the original bulk structure into a sheet-like structure, and its subsequent application as a cathode material for lithium-sulfur batteries.

[0065] To prove the performance of the hierarchical graphite nanosheets as the cathode material for lithium-sulfur batteries, lithium-sulfur batteries were assembled for electrochemical performance testing. Meanwhile, to prove the influence of the two-step activation method in Step 3 on the performance, lithium-sulfur batteries were assembled with the recycled graphite obtained in Step 2 as the cathode material of the lithium-sulfur battery for electrochemical performance testing.

[0066] It should be noted that the battery assembled with the recycled graphite obtained in Step 2 of Example 1 as the cathode material is named Battery S1; the battery assembled with the hierarchical graphite nanosheets obtained in Step 3 of Example 1 as the cathode material is named Battery S2.

[0067] The method for assembling the lithium-sulfur battery is a conventional method, which is specifically as follows: First, the sample to be tested and sulfur powder are ball-milled. The ball-milling conditions are that the mass ratio of the sample to be tested to sulfur powder is 1:3, the ball-to-material ratio is 150:1, the ball-milling speed is 200 rpm, and the ball-milling time is 1 h. After ball-milling, a carbon-sulfur composite material is obtained by the conventional melt diffusion method. Then, using the carbon-sulfur composite material as the active material, conductive carbon black as the conductive agent, polyvinylidene fluoride as the binder, and N-methylpyrrolidone as the solvent, a slurry is prepared with the mass ratio of the active material, conductive agent, and binder being 8:1:1. Finally, the slurry is uniformly coated on the aluminum foil to obtain an electrode sheet, and a button battery is assembled by adding a traditional lithium-sulfur battery electrolyte in a glove box.

[0068] The cyclic voltammetry test results of Battery S2 are as Figure 2 shown. There are two cathodic peaks at 2.36 V and 2.03 V, and the main charge peak at 2.34 V and the small peak at 2.38 V are anodic peaks. Moreover, the first three cycles of the CV curve overlap well and the positions of the charge-discharge peaks do not change significantly. The test results show that the hierarchical graphite nanosheets exhibit good electrochemical reversibility and cycling stability during cycling. In addition, the sharp peaks also indicate that the sulfur in the composite material has good reactivity, showing fast reaction kinetics characteristics.

[0069] The cyclic charge-discharge tests were carried out on Battery S1 and Battery S2 respectively.

[0070] The test results of Battery S1 are as Figure 3 shown. The initial discharge specific capacity of the recycled graphite prepared in Step 2 is 682 mAh·g -1 , and after 100 charge-discharge cycles, it remains 405 mAh·g -1 , and the capacity retention rate is 59.4%.

[0071] The test results of Battery S2 are as Figure 3 shown. The hierarchical graphite nanosheets prepared by the two-step activation method provide an initial discharge specific capacity of 1410 mAh·g at a current density of 0.1 C -1 , and after 100 charge-discharge cycles, it remains 841 mAh·g-1 , the capacity retention rate is 59.6%.

[0072] By comparing the cyclic charge-discharge test results of hierarchical graphite nanosheets and recycled graphite, it can be seen that sulfur has better electrochemical activity on hierarchical graphite nanosheets, corresponding to the sharp peak shape obtained from cyclic voltammetry tests.

[0073] To prove the influence of the two-step activation method on the material properties, that is, the role played by each activation method in the technical solution, Comparative Example 1 and Comparative Example 2 are provided, which are waste graphite recycling-activation methods that only use the calcination activation method and the solvothermal activation method respectively.

[0074] Comparative Example 1

[0075] A waste graphite recycling-activation method based on the calcination activation method, the steps not specifically described are the same as those in Example 1, except that: in Step 3, the mixed reaction solution obtained from recycled graphite and potassium hydroxide is not subjected to the first-step solvothermal activation operation, and is directly subjected to air drying to obtain a mixture, and the subsequent second-step calcination activation and post-treatment are carried out. The obtained product is named recycled graphite by the calcination activation method.

[0076] The SEM test results of the recycled graphite by the calcination activation method are as Figure 5 shown. The recycled graphite by the calcination activation method has a lamellar structure. However, the graphite lamellae cannot be dispersed, and the graphite layer spacing has no obvious change. The test results show that without the first-step solvothermal activation, potassium hydroxide cannot enter the interior of the graphite block, directly resulting in only etching off part of the surface graphite during the second-step calcination activation.

[0077] The battery assembled with the recycled graphite by the calcination activation method obtained in Comparative Example 1 as the positive electrode material is named Battery D1. The cyclic charge-discharge test results of Battery D1 are as Figure 6 shown. It shows an initial discharge specific capacity of 973 mAh·g -1 at a current density of 0.1C. After 100 charge-discharge cycles, 503 mAh·g -1 remains, and the capacity retention rate is 51.7%.

[0078] By comparing Comparative Example 1 with Example 1, it can be seen that the initial discharge specific capacity is reduced by 437 mAh·g -1 . This result shows that in the two-step activation method of the present invention, the calcination activation method mainly acts on the high-temperature etching reaction between graphite and the activator potassium hydroxide.

[0079] Comparative Example 2

[0080] A method for recycling and activating waste graphite based on solvothermal activation method. The steps not specifically described are the same as those in Example 1, except that in step 3, the mixture obtained from the first-step solvothermal activation does not undergo the second-step calcination activation operation, but directly undergoes subsequent post-treatment, and the obtained product is named graphite recycled by solvothermal activation method.

[0081] The SEM test results of the graphite recycled by solvothermal activation method are as Figure 7 shown. The graphite recycled by solvothermal activation method shows a fluffy bulk structure, and a large amount of potassium hydroxide adheres to the surface. The test results indicate that during the solvothermal treatment, the etching reaction between graphite and the activator potassium hydroxide does not occur significantly, but rather makes the two fully mixed and uniform.

[0082] The battery assembled with the graphite recycled by solvothermal activation method obtained in Comparative Example 2 as the positive electrode material is named Battery D2. The cyclic charge-discharge test results of Battery D2 are as Figure 8 shown. It shows an initial discharge specific capacity of 569 mAh·g -1 at a current density of 0.1C, and remains 436 mAh·g -1 after 100 charge-discharge cycles, and the capacity retention rate is 76.6%.

[0083] By comparing Comparative Example 1 with Example 1, it can be seen that the initial discharge specific capacity is reduced by 841 mAh·g -1 . This result shows that in the two-step activation method described in the present invention, the solvothermal activation method mainly acts on the fusion of graphite and the activator potassium hydroxide. During the subsequent calcination activation treatment in the example, potassium hydroxide can react better with graphite and etch graphite from the inside, making its interlayer spacing larger, thereby realizing the preparation of hierarchical graphite nanosheets.

[0084] By combining and comparing Example 1 with Comparative Example 1 and Comparative Example 2, the following conclusions can be drawn: The two-step etching is indispensable in the preparation process of hierarchical graphite nanosheets. Although the calcination activation treatment can expose the graphite sheets, this etching from the outside to the inside cannot disperse the graphite sheets. After adding a pre-step solvothermal activation treatment, the graphite bulk becomes fluffy, and potassium hydroxide successfully enters the inside of the graphite sheets. During the subsequent calcination activation treatment, the etching reaction occurs simultaneously inside and outside, and thus hierarchical graphite nanosheets are successfully prepared.

Claims

1. A method for recycling and activating waste graphite, characterized in that It includes the following steps: Step 1: Conventional recycling of graphite. Place the negative electrode sheet of waste lithium-ion batteries in deionized water and perform ultrasonic treatment under certain conditions. After that, take out the copper foil peeled off from the negative electrode sheet, filter, wash, and dry it to obtain a graphite-conductive carbon-binder mixture containing graphite, conductive carbon, and binder. Step 2: Pyrolytic removal of conductive carbon and binder. Use pyrolysis to remove the binder. The specific method of pyrolysis is to pyrolyze the graphite-conductive carbon-binder mixture obtained in Step 1 under an argon atmosphere under certain conditions to obtain recycled graphite. Step 3: Two-step activation method for recycled graphite. Add the recycled graphite obtained in Step 2 and potassium hydroxide to deionized water to obtain a mixed reaction solution. Then, ultrasonically treat the mixed reaction solution under the condition that the ultrasonic time is 2 h and perform the first-step solvothermal activation. The specific method of the first-step solvothermal activation is to perform a solvothermal reaction under certain conditions. After that, perform blast drying on the solvothermal reaction product under certain conditions to obtain a mixture. After that, perform the second-step calcination activation on the mixture. The specific method of the second-step calcination activation is to calcine under certain conditions under an inert gas condition. Step 4: Post-treatment of the calcination reaction product, that is, preparation of hierarchical graphite nanosheets. The specific method of the post-treatment is to first perform ultrasonic treatment and then stirring in a nitric acid solution. After the post-treatment, perform centrifugation and drying to obtain hierarchical graphite nanosheets.

2. The method according to claim 1, characterized in that: The conditions for the ultrasonic treatment in Step 1 are that the ultrasonic power is 700 - 1200 W and the ultrasonic time is 1 - 5 h.

3. The method according to claim 1, characterized in that: The conditions for the pyrolysis in Step 2 are that the heating rate is 5 - 10 °C / min, the pyrolysis temperature is 300 - 600 °C, and the pyrolysis time is 1 - 5 h.

4. The method according to claim 1, characterized in that: The mass ratio of the recycled graphite, potassium hydroxide, and deionized water in Step 3 is 1:2:

500. The conditions for the first-step solvothermal activation in Step 3 are that the reaction temperature is 120 - 180 °C and the reaction time is 5 - 24 h. The conditions for the blast drying are that the drying temperature is 100 - 200 °C and the drying time is 5 - 24 h.

5. The method according to claim 1, characterized in that: The conditions for the second-step calcination activation in Step 3 are that the heating rate is 5 - 10 °C / min, the calcination temperature is 500 - 900 °C, and the calcination time is 1 - 5 h.

6. The method according to claim 1, characterized in that: The conditions for the post-treatment in Step 4 are that the concentration of the nitric acid solution is 8 - 12 mol / L, the ultrasonic time is 5 - 60 min, and the stirring time is 1 - 12 h.

7. The method according to claim 1, characterized in that: The obtained layered graphite nanosheets are used as a sulfur carrier for the positive electrode material of a lithium-sulfur battery, and are characterized in that: the initial capacity at a current density of 0.1C is 1410 mAh·g -1 .

8. The method according to claim 1, characterized in that: The obtained layered graphite nanosheets are used as a sulfur carrier for the positive electrode material of a lithium-sulfur battery, and are characterized in that: after 100 charge-discharge cycles, the remaining capacity is 841 mAh·g -1 , and the capacity retention rate after 100 charge-discharge cycles is 59.6%.

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